By setting the FieldMap.PopupIsDefault property true, when a user selects an action on a row, the resulting page will be displayed in a popup instead of changing the complete web page.
The popup view of the next screen displayed uses the Enhanced UI DHTML windowing framework, which is based on iFrames, not new browser windows. This provides a more natural windowing environment as well as avoiding any issues with popup blockers.
Typically, when a standard action is entered, for example, a Single-Click selection, the entire page will be replaced with the displayed screen.
By setting PopupIsDefault on the FieldMap, you can enable a user being able to view multiple actions in popups. This lets the user act on one row, close the popup and act on another row.